A thruster as here understood is a steerable propulsion device arranged mainly beneath the hull of a marine vessel (see FIG. 1). The thruster is formed of a propeller unit (rotatable/steerable round a vertical axis) beneath the hull and of a substantially vertical housing. The vertical housing extends up into the hull of the marine vessel through an opening at the bottom of the hull. The circumference of the opening is provided with means, for instance bearings, for holding the upper end of the vertical housing within the hull. The upper end of the vertical housing is provided with a first gear wheel, which communicates with one or more smaller second gear wheels each rotated by a hydraulic steering motor. The first and second gear wheels form the mechanical components of a gear transmission azimuth steering arrangement. The vertical housing of the thruster is in itself rotatable by means of the first gear wheel and the hydraulic steering motor/s attached to a non-rotary support frame, which supports the thruster to the opening at the bottom of the hull. The drive of the propeller is arranged through the hollow interior of the vertical housing. Thus the drive may be mechanical with drive shafts and angular gears. Naturally the drive of the propeller may also be arranged hydraulically or electrically.
Normally the steering arrangement of a thruster is intended to control the azimuth angle of the unit. The positioning of the mechanical thruster part is done by one or more hydraulic motors. JP52-77397 (Kawasaki Heavy Industries) for instance discusses a traditional hydraulic steering arrangement of a thruster. A proportional directional valve controls the oil flow from the hydraulic pump to the hydraulic steering motor. Once the correct azimuth angle is reached, the proportional valve is used to close the flow connection from the pump to the hydraulic motor as well as from the hydraulic motor back to the oil tank. The azimuth angle of the thruster is then maintained as both flow paths from the hydraulic motor are closed and the motor is, thus, not able to rotate.
A somewhat more detailed illustration of a hydraulic steering arrangement of a thruster is shown in FIG. 2. The major difference when compared to the steering arrangement of the JP document 52-77397 is the counterbalance block, which is arranged between the pilot-operated proportional directional valve and the hydraulic steering motor turning the thruster. The counterbalance block includes a safety valve arrangement and two, i.e. a first and a second, counterbalance valve arrangements. The purpose of the safety valve is to open a flow path from one port of the hydraulic motor, if the pressure at this port exceeds a predefined allowable value. The pressurized oil flows to the safety valve, opens the valve at the predefined set point of the safety valve, and passes to the pipe of the hydraulic motor prevailing at a lower pressure, or returns from the outlet port to the tank.
The counterbalance valve arrangements have been coupled to the hydraulic pipes connected to the ports of the hydraulic motor farther away than the safety valve arrangement. The counterbalance valve arrangement includes a check valve and a hydraulically operated pressure relief valve. The purpose of the counterbalance valve arrangements is to lock the steering i.e. maintain the thruster in the direction it has been turned by the proportional valve with the help of the hydraulic steering motor/s. At a steering phase the counterbalance valve arrangements function as follows. The first valve arrangement positioned at the inlet pipe of the steering motor/s allows the pressurized oil to flow via the check valve to the inlet port of the steering motor/s with a minimal pressure loss. In the second valve arrangement at the outlet pipe of the steering motor/s the pressure of the returning oil affects the pressure relief valve and opens it together with the pilot pressure from the pressure pipe between the proportional directional valve and the first counterbalance valve arrangement. Thus the returning oil has a certain counter pressure, i.e. a pressure loss takes place in the second counterbalance valve. When the steering action is stopped for maintaining the thruster at its direction, the proportional directional valve is moved to its centre position whereby the proportional valve forms a connection from the counter balance valves to the tank. Thus the connection from the pump to the counterbalance valve is closed. In such a case the steering motor/s is/are subjected to no internal load. However, the thruster may be subjected to external loads from the sea, whereby the thruster acts on the steering motor/s and tries to rotate such. In practice this means that the motor/s start/s acting as pump/s. The motor/s create/s oil pressure that acts on both the safety valve and one of the counterbalance valves of the counterbalance block. As long as the hydraulic motor/s has/have no internal leakage the thruster is not able to turn until the pressure exceeds the predetermined value required to open the safety or counterbalance valve. When the value is exceeded the pressurized oil flows from the outlet port of the steering motor/s to the inlet port/s or to the tank.
However, now that the thrusters have gained acceptance in marine vessels that are used in arctic conditions, too, the loads, and especially the torque ice subjects to thrusters have to be taken in careful consideration. The torque exerted by ice loads on the thrusters may reach an unacceptable magnitude for the structure of the thrusters or the entire steering arrangement. As discussed above the prior art hydraulic steering arrangements have safety valves aiming at taking care of the oil flows and pressures related to hydrodynamic loading with a limited safety margin only.
However, the prior art safety valves have not been designed and positioned in view of the sudden loads created by hard and large solid objects like for instance ice. The loads associated with contact with ice have a profound dynamic character. A typical aspect is that these loads can lead in a very short time to a torque load on the steering system above a value the construction can accommodate. Also if the vessel containing the thruster is sailing at a certain speed at the moment the thruster comes in contact with ice the speed at which the ice block (or other large solid object) tries to rotate the thruster is easily 5 to 10 times the normal steering speed. The forced rotation of the thruster results in the hydraulic motors acting as pumps and generating a flow significantly larger, also 5 to 10 times, than the one required for steering purposes. The prior art safety valves have been selected to accommodate a flow corresponding to the normal steering speed upon blockage of the steering system. The same is true for the dimensioning of the oil pipes between the hydraulic motors and the safety valve. Also the positioning of the valves arranged at, possibly, a considerable distance from the steering motor/s has been based on the requirements associated with the steering purposes. In other words, the main design parameter has been the requirement of the steering of the thruster that its speed of rotation is of the order of 2 rpm whereby the oil pipes to and from the steering motor/s should be able to handle such flows. On the one hand, when the hydraulic steering system is subjected to the significantly increased flows as generated by the hydraulic motors the limited diameter of the piping results in high flow velocities within the pipes, whereby the oil friction in the pipes creates a substantial pressure loss within the entire hydraulic pipeline between the hydraulic motor and the safety valve. The maximum pressure within the steering system is limited by the maximum mechanical strength of the steering system or thruster with respect to the torque load. A higher load leads to damage of the steering system or the thruster. Given the maximum allowable torque or pressure on the steering system, the pressure loss within the piping would require a lower setting of the safety valve. In other words, the higher rotational speeds of the thruster are desired to be taken into account, the lower the opening pressure of the safety valve should be. The lowering of the opening pressure of the safety valve would however interfere with the normal steering operation. In a similar manner the quick rotation of the hydraulic motor/s as pump/s creates in the inlet port of the steering motor/s reduced pressure, which easily leads to non-controlled cavitation, as the pipes leading to the inlet port/s of the hydraulic motor/s do not allow oil to flow quickly enough into the motor/s.
On the other hand, if the hydraulic pipes and safety valve are sized with respect to the maximal imaginable flow, and the distance between the hydraulic motor/s and the counterbalance block, where the pressure relief valve is positioned, is several meters, then the hydraulic pipelines are relative larger volumes. Those volumes may introduce undesired dynamic behavior such as pressure pulses and time delays. Especially in case of large external loads, which appear as impact loads, the undesired dynamic phenomena may occur. Pressure pulses are highly undesired as they can cause chattering of the pressure relief valve, i.e. the valve opens and closes in very short time periods. This may result in damage for the pressure relief valve and hydraulic motor. Also, for normal steering operations, pressure pulses and time delay may influence the steering behavior in a negative way such that the steering movement is non-smo.